The present invention relates to LTE-Advanced system which will be part of 3GPP LTE Release 12. More specifically, the present invention focuses on small cell ON/OFF switching enhancement and related discovery procedures. “Small Cell Enhancements—Physical-layer Aspects” Work Item is currently ongoing in 3GPP and is lead by RAN WG1 (Work Item description (WID) in document [1]). One of the topics discussed intensively is the small cell on/off operation: the basic idea is to facilitate on/off switching of the small cells to e.g. reduce network energy consumption as well as interference during the times when the network load is low. The WID states the following:                “Discovery procedure/signal(s) are needed                    Cells operating a cell on/off may transmit discovery signal(s) supporting at least for cell identification, coarse time/frequency synchronization, intra-/inter-frequency RRM measurement of cells and QCL. (Note that QCL is not always necessary or possible depending on the procedure.)            This includes support of discovery and measurement enhancement(s) in DL and its usage in related procedures.”                        
The topic was further discussed in the 3GPP RAN1 meeting #76 with the following main agreements:                Discovery signals are used for RSRP and RSRQ—like RRM measurements        Discovery signals include PSS and one or more of (SSS, CRS, CSI-RS, PRS)        
The present invention relates to the discovery procedure and the related RRM measurements and related reporting mechanisms based on Discovery Signals (DS).
Based on the decisions and discussions in RAN1, the focus of small cell on/off and discovery enhancements is on semi-static time scales. More specifically, the work aims at improving the efficiency of currently available on/off mechanism by facilitating shorter transition times to/from eNB dormant (i.e. OFF) period. There are two primary ways of achieving this:                1. Enhancements to UE procedures related to cell activation/deactivation, handover etc. to reduce the transitions times between the ON and dormant states.        2. Transmission of DL discovery signals to facilitate timely discovery of dormant cells.        
The basic setting is illustrated in FIG. 1. During e.g. low network load, in a period 11 in which the cell is on, the network may decide to turn a cell off. The decision to turn the cell off is followed by an on-off transition period 12, during which the network empties the cells that are to be turned OFF from UEs, using e.g. handover, connection release, redirecting RRC_IDLE mode UEs to different frequency layers etc. Once the network is satisfied that there are no longer UEs camping on the cells to be turned OFF it may decide to turn the cell off and start a dormancy period 13. During the dormancy period 13, an eNodeB may still transmit (e.g. periodically) some DL signals to allow for the UEs supporting the dormant mode to discover and measure the dormant cell.
Further, when there is high network load, the network may decide to turn the cell on and after an Off-On transition period 14, the cell is ON during period 15.
The first above mentioned enhancement (enhanced UE procedures to reduce cell on/off transition times) generally falls outside of the scope of RAN1, and also outside of the scope of the present invention. However, the second enhancement (discovery signal) is of greater significance to 3GPP RAN1, and the present invention focuses on the discovery signals and the related UE measurements and reporting.
The scenario in which the UE needs to perform RRM measurements based on discovery signals (DS) may comprise three types of cells even on the same frequency layer, as indicated in FIG. 2.                1. Legacy cells 21 not applying ON/OFF and not transmitting any discovery signals (apart from PSS/SSS/CRS)        2. Rel-12 cells 22 transmitting discovery signals and that are turned OFF        3. Rel-12 cells 23 transmitting discovery signals and that are turned ON        
The UE should report to the eNodeB the RRM measurements (namely RSRP & RSRQ) that are needed for making the decision on which cell the UE should be handed over to, or in the case of carrier aggregation, which cell(s) should be configured and/or activated to the UE as an SCell. In order to assist in the network in this decision making, several measurement events have been defined in E-UTRAN. The current definition (as of Rel-11) of the measurement events are as follows:                Event A1: Serving (cell) becomes better than threshold        Event A2: Serving (cell) becomes worse than threshold        Event A3: Neighbour (cell) becomes offset better than PCell        Event A4: Neighbour (cell) becomes better than threshold        Event A5: PCell becomes worse than threshold1 and neighbour (cell) becomes better than threshold2        Event A6: Neighbour (cell) becomes offset better than SCell        Event B1: Inter RAT neighbour (cell) becomes better than threshold        Event B2: PCell becomes worse than threshold1 and inter RAT neighbour (cell) becomes better than threshold2        
The problem with event based reporting in the context of small cell ON/OFF operation is that the events do not consider the ON/OFF state of the cell. Instead, all cells use the same reporting criteria and trigger reports in the same way. Also, from network point of view, it can make sense to e.g. prioritize cells that are already ON in the selection of the cell for a given UE, even if another (OFF) cell would have slightly higher RSRP/RSRQ. It may also be desirable to have the events trigger differently for ON and OFF cells (e.g. it might be desirable that a report for an OFF-cell triggers more slowly than for an ON-cell). In this way, the additional interference and energy consumption resulting from a small cell being turned on can be avoided, when possible. Present measurement definitions do not permit for this kind of prioritization since eNB does not obtain information from the cell activity status, or cannot make the UE report cells differently based on their activation status. The current specifications do admit some freedom in e.g. allowing a different cell-specific offsets for a group of cells or different time-to-trigger for a group cells, but these methods do not consider the ON/OFF-state of a cell, either (since it hasn't existed before this).
Another problem related to DS based RRM measurements results from the design of the DS (which is currently open in the 3GPP). It has been agreed in RAN1#76 meeting that DS shall include at least PSS and additionally one or more of SSS/CRS/CSI-RS/PRS. However, on a legacy carrier, RRM measurements and cell discovery is always based on PSS/SSS/CRS. If the DS agreed for Rel-12 in 3GPP consists of signals other than the ones used in the legacy case, the accuracy and the performance of the measurements will inevitable be different (maybe better, maybe worse) from that of when using legacy measurement procedures. This also means that with the current criteria, ON-cells and OFF-cells could potentially be compared against each other even if their measurement results are based on different reference symbols or e.g. different L1/L3 filtering mechanism or filter length. Furthermore, the network may also want to prioritize or deprioritize e.g. UE reporting of legacy cells to maximize the benefits of small cell ON/OFF operation.
Document [5] has mainly addressed the structure and the configuration of Discovery Signals. For example, this document discusses the option of having the discovery signal based on PSS/SSS/CRS, and the details of configuring them.
The feature of Target Cell-specific TTT has been approved to Rel-12 in the context of Hetnet Mobility WI—see document [2].
The Cell Individual Offset (CIO) has been specified to LTE in Rel-8, and has existed in UMTS before that. See document [3] for more details.
There has been no discussion in the 3GPP so far related to the details of RRM measurement reporting for DS based RRM measurements. The RAN1 “LS on small cell on/off and discovery” (cf. document [4]) was briefly discussed in the RAN2#85 meeting but no actions were taken so far to consider the impacts to RAN2 aspects.